Zwitterionic Surfactant Micelle-Directed Self-Assembly of Eu-Containing Polyoxometalate into Organized Nanobelts with Improved Emission and pH Responsiveness.

Abstract

Recently, hybrid coassembly between polyoxometalates (POMs) and cationic building blocks provides an efficient strategy to greatly optimize POMs' functionality as well as their aggregate structural diversity. Adaptive hybrid supramolecular materials with enhanced luminescence have then been obtained from lanthanide-containing POMs. In this work, a commercially available and pH-switchable zwitterionic surfactant, tetradecyldimethylamine oxide (C14DMAO), was chosen to coassemble with a lanthanide-containing anionic POM [Na9(EuW10O36)·32H2O, abbreviated as EuW10] in water. The much improved red-emitting luminescent nanobelts at a C14DMAO/EuW10 molar ratio ( R) of 20 were obtained, which exhibited longer luminescence lifetime and higher quantum yield compared with EuW10 aqueous solution. After careful characterization of morphology and structure of nanobelts, an unusual axial lamellar aggregation arrangement mechanism was proposed. It was the partial protonation of C14DMAO at the solution pH of about 6.5 that led to positively charged micelles, being bridged by anionic EuW10 clusters to aggregate into such novel nanobelts under the synergetic effects of appropriate electrostatic, hydrogen-bonding, and hydrophobic interactions. The resulted pH-responsive luminescent nanobelts and their aggregation model should offer attractive references for preparing smart optical supramolecular materials.